Understand the idea.
Around and inside
The boundary is the line around a flat shape. The interior is the space inside it. Trace the rectangle’s outline to follow its boundary; shade the region enclosed by that outline to show its interior.
A fence follows the edge of a garden, while grass covers the ground inside. Deciding which part matters helps you choose a measurement before using any numbers.
Choose the useful measure
Perimeter is the total distance around the boundary, measured in length units such as centimetres. Area is the amount of surface covered inside, measured in equal square units. The two measures answer different questions.
Ask whether the task needs a length around an edge or a covering over a surface. Adding the sides of a rectangle finds its boundary length; counting the square units inside finds how much space it covers.
See the method in action.
A frame
A photo needs a frame around its edge. Do we need its perimeter or its area?
Worked solution
- 1
Identify where the frame goes.
The frame follows the photo’s outside edge rather than covering the photo.
- 2
Choose the matching measure.
The full distance around an edge is its perimeter, so perimeter tells us the required frame length.
Perimeter.
A covered board
A rectangular board is 4 unit squares across and 3 squares down. How much space does it cover?
Worked solution
- 1
Count the squares in one row.
The grid has four equal unit squares across each row.
- 2
Count all three rows.
Adding the three rows counts every square once, with no gaps or overlaps.
- 3
State the covered area.
The board covers twelve unit squares, so this is an area measurement.
12 square units.
Try it yourself.
Check your understanding
Would you measure perimeter or area to buy a carpet?
Show answer & reasoning
Area.
Choose area because carpet covers the floor surface. Perimeter would measure the distance around the room’s edge rather than the amount of carpet needed.
Keep practising
6 worksheet questions · 7 marks
Read the questions online
- A garden needs edging all the way around. Which measure is needed?1 mark
- A wall will be painted. Which measure describes the surface to paint?1 mark
- Name the part of a shape that separates its inside from its outside.1 mark
- Which unit fits an area: centimetres or square centimetres?1 mark
- A 3 by 2 grid is completely shaded. Find its area.1 mark
- A student says, “The area of a square is the length of its fence.” Explain the error.2 marks
Read the worked solutions
Equivalent correct methods earn the corresponding marks. Include units where asked.
- A garden needs edging all the way around. Which measure is needed?1 mark
Edging follows the whole boundary.
Perimeter. - A wall will be painted. Which measure describes the surface to paint?1 mark
Paint covers the wall interior.
Area. - Name the part of a shape that separates its inside from its outside.1 mark
The boundary runs around the edge.
The boundary. - Which unit fits an area: centimetres or square centimetres?1 mark
Area counts square units that cover a surface.
Square centimetres. - A 3 by 2 grid is completely shaded. Find its area.1 mark
There are 3 squares in each of 2 rows.
square units.
6 square units. - A student says, “The area of a square is the length of its fence.” Explain the error.2 marks
A fence follows the boundary rather than covering the surface.
The fence length is perimeter; area is the space inside.
Curriculum reference & lesson scope
Australian Curriculum v9 / SA Curriculum Mathematics 2026
Boundary/interior; measuring perimeter; covering/counting/estimating area; comparing shapes.
Official curriculum source ↗